The present invention relates to the field of chemical testing of intermetallic materials, more specifically for testing intermetallic compound of nickel and titanium commonly known as Nitinol for surface inclusions and lack of homogeneousness by immersing it in aqueous solution of sodium hypochlorite (NaClO) and checking for black flocculent precipitate developing on the particular surface site.
The site or sites where this black flocculent precipitate start to develop is the place of surface inclusion and give evidence of lack of homogeneousness of Nitinol material, which is sign of inferiority and should be the base for rejection of such raw material or finished product.
The surface inclusions are causing the maximum stress during bending-rotation and flexing especially peripheral stents and lead to their fracture. Also inclusions are sources by themselves of sites were corrosion starts to dissolve matrix materials releasing nickel ions harmful to living cells surrounding particular implantable device.
The Nitinol inclusions can be classified in two ways: by its origin and by its chemical composition. The classification by origin giving us two kinds of inclusions: native which originate during production of bulk material and another one foreign introduce during finishing operations.
The native inclusions are randomly distributed through whole volume of material and finding them on the surface is the excellent indicator of quality of the bulk of material. In contrast foreign inclusions are strictly surface phenomenon introduced to the surface during finishing operations as: glass-bead, sand or aluminum oxide blasting, heat treatment, mechanical polishing, lapping, laser cutting, drawing, electro discharge machining etc.
The classification by chemical composition is more complicated. Taking under consideration the very small size of inclusions the chemical analysis is often difficult and very often leads to error. Those inclusions could be broadly classified as carbides (TiC), oxides (Ti4N2Ox, TiO2) or intermetallic precipitates (Ni4Ti3).
It is widely recognized that carbides are created during (VIM) vacuum induction melting from carbon crucibles used in this process. On another hand oxides are originate in bigger amount and in larger particles size during (VAM) vacuum arc melting. The third process which claims four to ten times lower carbon content due to use of water cooled crucibles is (EBM) electron beam melting.
But regardless of all of the Nitinol production methods mentioned above, none of them is perfect and in the present time it isn't possible to produce 100% inclusions free, homogeneous Nitinol.
Till now the only way to check up the Nitinol surface for inclusions were microscopic and instrumental methods as: scanning electron microscope (SEM) with energy dispersive X-ray (EDX) spectrometry, atomic force microscopy (AFM), transmission electron microscopy (TEM) X-ray diffraction, Auger spectrometer with back-scatter electron detector (BSE).
All of those above mentioned techniques are very expensive, time consuming, demanding highly trained operators and by this they are excellent techniques for scientific research or limited industrial inspections, but unpractical for large scale production inspections.
Because of this the present invention which is very simple, inexpensive, very effective (almost 100%), doesn't require expensive instrumentation and highly trained operators and can be applied to raw materials as well to finished products are perfectly suited for mass inspections.
In the event of positive test results of raw material intended for production, material can be rejected before even starting expensive manufacturing processes and by this money and time can be saved.
The post production test of finished products can eliminate defected products (which have avoided detection during raw material testing, because inclusions were not present on the surface during initial test and appear on the surface as a results of production operations: removing excess material by mechanical, chemical or electrochemical processes and revealing inclusions from the bulk of material or by introducing externally new inclusions to the surface of finished products as a result of manufacturing operations as laser cutting, sand blasting, drawing etc.
By applying post production testing lot of very serious (fracture of endodontic rotary file for example) and even death threatening problems (as for example fracture of carotid stent or neurovascular coil used to treat brain aneurysm) could be avoided.